Recently, there has been an increasing interest in energy storage technology. Batteries have been widely used as energy sources in the fields of cellular phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, along with the trend directed to smaller and lighter electronic devices, development of rechargeable secondary batteries has been focused on a secondary battery as a small, light, large-capacity rechargeable battery.
Electrolyte used for electrochemical devices such as batteries and electric double layer capacitors using electrochemical reaction was mainly a liquid electrolyte, particularly an ion conductive organic liquid electrolyte obtained by dissolving salt into a non-aqueous organic solvent.
However, if such a liquid electrolyte is used, there arise problems in stability since electrode materials may be degraded, the organic solvent may be volatilized, and combustion may occur due to the temperature increase of surroundings or battery itself. In particular, a lithium secondary battery generates gas inside the battery due to decomposition of carbonate organic solvent and/or side reaction between the organic solvent and the battery while the battery is charged or discharged, so the thickness of the battery may be expanded. In addition, at high temperature storage, such reaction is accelerated to further increase an amount of generated gas.
The gas continuously generated as mentioned above causes increase of inner pressure of the battery, which may deform a center of a certain surface of the battery, for example making an angled battery be inflated in a certain direction. In addition, there happens a local difference in adherence on an electrode surface inside the battery, so the electrode reaction may not occur identically in the entire electrode surface. Thus, it is inevitable to cause deterioration of performance and stability of the battery.
Generally, the stability of battery is improved in the order of liquid electrolyte<gel polymer electrolyte<solid polymer electrolyte, but it is also known that the performance of battery is decreased in that order. Due to the inferior battery performance as mentioned above, it is known that batteries adopting solid polymer electrolyte are not yet put into the market.
Meanwhile, the gel polymer electrolyte may keep a thickness of battery constantly since it ensures excellent electric and chemical stability as mentioned above, and also the gel polymer electrolyte may allow to make a thin film battery since it ensures excellent contact between electrode and electrolyte due to inherent adhesive force of the gel.
The gel polymer electrolyte may be prepared in a state that an electrolyte solvent is impregnated among polymer matrix. At this time, properties of a gel polymer secondary battery may be changed depending on the used electrolyte solvent.
Ethylene carbonate frequently used as an electrolyte solvent of the gel polymer electrolyte ensures excellent dissociation and transfer of lithium ions due to high polarity, but it shows a problem of deteriorated low temperature performance of a secondary battery due to a high freezing point.
In addition, in case carbonate such as ethylene carbonate or propylene carbonate and ring ester such as gamma-butyrolactone (GBL) are used in mixture as an electrolyte solvent, low temperature performance of a secondary battery may be improved since the electrolyte solvent has a low freezing point, but there may be deterioration of battery performance since the ring ester shows bad impregnation with respect to electrode or separator and also dissolves LiPF6 mainly used as an electrolyte salt.